UV stabilizers (the terms UV stabilizer and UV absorber are synonymous within this specification) for polymers are known within the prior art and in essence derive from the following groups of substances: triazines, benzotriazoles, benzophenones, sterically hindered amines (Hals=hindered amine light stabilizer), and from many other groups. An example of a list is found in DE-A-10135795, (whose United States equivalent is United States Patent Application Publication No. 2002/083641A1). The UV stabilizers of interest here have the task of protecting the polyester from the effects of UV radiation, and they moreover also provide protection from UV light to the products located behind the film in the path of the radiation. UV-stabilized, oriented polyester films are known, and the particular effectiveness of stabilizers from the triazines class has likewise been described, for example in WO-A-1998/06575.
A disadvantage common to most of those members of the abovementioned absorber groups that are suitable for polyesters is that their absorption decreases sharply as wavelength rises in the range from 360-400 nm, and concentrations that are of interest commercially (up to a maximum of 5% by weight) generally lead to transparency >40% above wavelengths as low as 390 nm in unfilled clear polyester films (<0.5% particle-fill level) (transparency and transmittance being synonymous within this specification). The region above 320 nm is of no importance for stabilizing polyethylene terephthalate film itself, since polyethylene terephthalate has ceased to absorb at that wavelength; even film comprising polyethylene naphthalate has almost no remaining absorption above 360 nm, and is therefore not damaged by light of those wavelengths. However, the position is different in the case of many other organic materials, such as printing inks, vitamins, fats, flavors, natural dyes, etc. Some of these absorb very strongly in the range from 360 to 400 nm and correspondingly are subject to severe damage caused by these wavelengths. It is therefore desirable to protect substances of this type by using films which provide maximum shielding in said range. In contrast, strong absorption by the film above 400 nm is mostly not desired, since this gives the film an undesired coloring.
Substituted naphthalene-1,8-dicarboxylic monoimides are a relatively new class of commercially available UV absorbers. Compounds of this type, and their use in polyesters, have been described inter alia in WO-A-2005/047265 A1, whose United States equivalents are United States Application Publication No 2005/0171252 A1; United States Application Publication No. 2007/0100033 A1; and U.S. Pat. No. 7,714,044 B2. Each of WO-A-2005/047265 A1; United States Application Publication No. 2005/0171252 A1; United States Application Publication No. 2007/0100033 A1; and U.S. Pat. No. 7,714,044 B2 is incorporated herein by reference in its entirety.
Said specification describes the use of naphthalene-1,8-dicarboxylic monoimides alone or in combination with other UV absorbers. UV absorbers that may be mentioned for additional use are benzotriazoles, triazines and diphenylcyanoacrylates. Other classes of stabilizer, e.g. benzoxazines, are not mentioned, but a plurality of compounds from the sector of the benzotriazoles and diphenylcyanoacrylates are mentioned as suitable. Examples mentioned are not only 100% of naphthalene-1,8-dicarboxylic monoimide in examples 25-27 but also a combination with diphenylcyanoacrylate in example 28.
Napthalene-1,8-dicarboxylic monoimides of structure I-C (in particular I-C.1-10) in WO-A-2005/047265 have markedly higher absorption concentrations of from 0.1 to 5% by weight in polyester films of thickness from 8 to 200 μm than most other known commercially available UV stabilizers from the triazines, benzotriazoles, benzophenones, benzoxazines, or diphenylcyanoacrylates groups. However, their absorption decreases markedly below 350 nm, and since the intrinsic absorption of polyester films of said thickness makes a significant contribution only below 320 nm, there is a resultant unprotected gap within the UV-A region.
Furthermore, compounds of the structure I-C of WO-A-2005/047265 exhibit an intensive bluish violet fluorescence in polyester films. This is actually mentioned in WO-A-2005/047265 and is described as favorable for yellowness index reduction. However, this fluorescence is unwanted in, or even prevents use of, polyester films for food-and drink-packaging, and this also applies to films for other applications which involve some degree of printing, because it generates an extremely “synthetic” impression, and when the average user who “dislikes chemicals” sees this he/she can be reluctant to purchase. In printed applications, the fluorescence causes a color shift or an unwanted “color” in the unprinted regions.